actually the more rubies you have the more max stacks you can get,it's not like with mages and that electrtic buff that the more rubies you have the more stacks you get per attack
with 1 ruby max 3 stacks
with 2 rubies max 6
with all max9
(S)He's not talking about Unison,but about Grand Finale itself.
Grand Finale's rubies don't give you 1 more attack per ruby,but they add 1 attack(r1) and then they raise that final attack's damage(r2,r3).
So it's not a bug,it just works like that lol,since there's written:"Your successful Dancing Blade and Raving Blade spell casts ONE additional attack for X to Y physical damage",where X&Y depend on your gear and the rank of the skill.